1. Field of the Invention
The present invention relates to a permanent magnet assisted (hereinafter, referred to as “PMA”) synchronous reluctance motor (hereinafter, referred to as “synRM”) and a method for imposing a magnetic force thereon, and more particularly to a PMA synRM, which is connected to a compressor, and comprises a sufficiently large number of flux barriers formed in a rotor and a decreased number of magnets inserted into the flux barriers, and a method for imposing a magnetic force thereon.
2. Description of the Related Art
Now, with reference to FIGS. 1 and 2, the structures of a conventional synRM and a conventional PMA synRM will be described.
FIG. 1 is a partial cross-sectional view of the conventional synRM. As shown in FIG. 1, the conventional synRM comprises a stator 1, and a rotor 2 rotatably installed in the stator 1 and synchronously rotated to minimize the magnetic reluctance by a reluctance torque generated due to the interaction between the stator 1 and the rotor 2 when power is applied to the stator 1.
A plurality of flux barriers 3 for blocking the current of the magnetic flux are formed along the radius of the stator 2, and a rib 4 for facilitating the smooth current of magnetic flux is placed in a space between the ends of the flux barriers 3 and the outer diameter of the stator 1.
Hereinafter, the operation of the above-described conventional synRM will be described in detail.
When power is applied to coils (not shown) wound on the stator 1, a rotating magnetic field is formed. The rotor 2 generates induced current due to the rotating magnetic field, thereby being magnetized. Since the current of the magnetic flux, which flows in the direction of passing through the flux barriers 3 containing an air layer therein, is blocked by the flux barriers 3, the magnetic reluctance is increased in the direction toward the flux barriers 3.
The rib 4 placed between the flux barriers 3 and the stator 1 is obtained by stacking steel sheets, having a low thickness, which are generally the same material as that of the rotor 2. Thus, since the current of the magnetic flux toward the rib 4 is not blocked, the magnetic reluctance toward the rib 4 is decreased.
Accordingly, the current of the magnetic flux of the stator 2 has variations in magnetic reluctance according to routes. The larger the difference of the magnetic reluctance is, the smaller current generates a high reluctance torque.
FIG. 2 is a partial cross-sectional view of a conventional PMA synRM. The conventional PMA synRM is obtained by inserting permanent magnets into the insides of the flux barriers 3 formed in the rotor 2 of the conventional synRM. Differing from the conventional synRM, the conventional PMA synRM uses magnetic torque as well as reluctance torque, thereby having an improved efficiency.
Hereinafter, the constitution and operation of the above-described conventional PMA synRM will be described in detail.
The conventional PMA synRM comprises the stator 1, the rotor 2 rotatably installed in the stator 1 and provided with the flux barriers 3 formed therein, and magnets 3a inserted into the flux barriers 3 in a predetermined direction along the radius of the rotor 2.
The above PMA synRM, compared to a general synRM, has a decreased inductance (Lq) along the q axis due to the energy (λm) of the magnetic flux generated by the magnets 3a inserted into the flux barriers 3 as well as the magnetic flux by power, and thus has an increased torque constant (Te). The comparison of the PMA synRM and the synRM will be described as follows.
            synRM                      Te        =                              3            2                    ⁢                      P            2                    ⁢                      {                                                            (                                                            L                      d                                        ⁢                                          i                      d                                                        )                                ⁢                                                                  ⁢                                  i                  q                                            -                                                (                                                            L                      q                                        ⁢                                          i                      q                                                        )                                ⁢                                                                  ⁢                                  i                  d                                                      }                                                  PMA        ⁢                                  ⁢        synRM                            Te        =                              3            2                    ⁢                      P            2                    ⁢                      {                                                            (                                                            L                      d                                        ⁢                                          i                      d                                                        )                                ⁢                                                                  ⁢                                  i                  q                                            +                                                (                                                            λ                      m                                        -                                                                  L                        q                                            ⁢                                              i                        q                                                                              )                                ⁢                                                                  ⁢                                  i                  d                                                      }                              
Here, d and q represent axes according to the positions of the rotor 2, and Te of the PMA synRM is a torque equation of the PMA synRM into which the magnets 3a are inserted through the q axis. As stated in the above equation, the additional magnetic flux generated due to the magnets 3a inserted along the q axis increases the torque of the PMA synRM at the same current.
Hereinafter, with reference to FIGS. 1 and 2, routes of the magnetic flux of the synRM and the PMA synRM will be comparatively described.
In the synRM as shown in FIG. 1, when the synRM is magnetized along the q axis, most of the magnetic flux flows through the rib 4, but a smaller portion of the magnetic flux passes through the flux barriers 3, this portion of the magnetic flux being referred to as a leakage magnetic flux. The leakage magnetic flux increases the value of the inductance (Lq) along the q axis, thereby decreasing the value of the torque.
In the PMA synRM as shown in FIG. 2, when the PMA synRM is magnetized along the q axis, the leakage magnetic flux generated due to the power applied to the stator 1 and the magnetic f lux generated due to the magnets 3a collide with each other. Thus, since most of the magnetic flux flows through the rib 4, the value of the inductance (Lq) along the q axis is decreased, thereby increasing the value of the torque.
In case that magnetic force is imposed on the magnets 3a of the PMA synRM and then the PMA synRM is connected to a compressor, foreign substances can stick to the rotor 2 of the PMA synRM. Accordingly, before a magnetic force is imposed on the magnets 3a of the PMA synRM, the PMA synRM must be inserted into and then connected to the compressor. Thereafter, a magnetic force is imposed on the magnets 3a of the PMA synRM by applying a high voltage to the coils wound on the stator 1.
In case that a small number of the flux barriers 3 are formed in the rotor 2, as shown in FIG. 2, the magnets 3a are inserted into all of the flux barriers 3, thereby allowing magnetic flux energy due to the voltage applied to the stator 1 to form a perfect magnetic flux route along the steel portion, the magnets 3a, and the steel portion of the rotor 2. Thus, it is possible to impose a magnetic force on the magnets 3a using the coils.
However, in case that a large number of the flux barriers 3 are formed in the rotor 2, the magnets 3a, which are inserted into all of the flux barriers 3, cause an increase of production costs of the PMA synRM.
In case that the number of the flux barrier formed in the rotor 2 is decreased so as to limit the increase of the production costs of the PMA synRM due to the use of a large number of the magnets, a reluctance torque constant is a low value when the PMA synRM is operated in a normal state, thereby decreasing the efficiency of the PMA synRM.